Dual action hydraulic control valve



Oct 1954 a. H. STICKNEY DUAL ACTION HYDRAULIC CONTROL VALVE Filed Feb. 27, 1953 2 Sheets-Sheet 1 IN VEN TOR. GEORGE STICKNEY I ATTORNE f;

'AND

1954 G. H. STICKNEY DUAL ACTION HYDRAULIC CONTROL VALVE 2 Sheets-Sheet 2 Filed Feb. 27, 1953 ow mm v 9 mm ODE m m AND ATTORNEYZ Patented Oct. 19, 1954 DUAL ACTION HYDRAULIC CONTROL VALVE George H. Stickney, Royal Oak, Mich, assignor, by mesne assignments, to Ford Motor Company, Dearborn, Micln, a corporation of Delaware Application February 27, 1953, Serial No. 339,206

6 Claims.

The present invention relates to a manually actuatable control valve for a fluid motor and more particularly to such a valve for a double acting fluid pressure actuated cylinder, the valve being effective to selectively actuate the cylinder, either in direct response to the extent of valve actuation or for a complete cylinder stroke Without the necessity of holding the valve open.

In the control of remotely located fluid pressure actuated cylinders, particularly for use in farm implements, it has been proposed that a control valve be provided which is effective to actuate the cylinder through an entire cylinder piston stroke to an extreme position by utilizing the back pressure from the cylinder piston to hold the valve in open position without the necessity of manually retaining the valve open. However, for the operation of a great number of implements, it is desirable to provide a valve which also may be utilized to accurate! 1y control movement of the cylinder piston, under complete manual control, to less than its full extent, so as to afford better positioning control for the implement to which the cylinder is attached.

The present invention provides an improved control valve which is operable in this fashion.

The valve is manually controlled through an actuating handle so that, when the handle is moved to an extreme position, the valve is effective to actuate the cylinder for a complete cylinder piston stroke, with the piston back pressure holding the valve in open position and to automatically return the valve to a centered position upon completion of the piston stroke. Thus, it is not necessary to hold the handle and the valve in open position to complete the cylinder stroke, nor is it necessary to actuate the valve to stop cylinder movement at the end of its stroke. If the handle is moved to a lesser extent, i. e., to less than an extreme position, the cylinder is actuated in accordance with the valve position and accurate manual control of the range of piston movement is obtained.

It is, therefore, an important object of the present invention to provide an improved manually actuatable control valve for a fluid motor.

Another object of the present invention is to provide an improved valve for a double acting fluid pressure actuated cylinder which is selectively manually actuatable to a holdover position eflective to cause complete movement of the cylinder through an entire range of movement, and to a manually adjusted position eifective to cause cylinder actuation under direct manual valve control.

A further important object is the provision of a control valve for a fluid motor having a valve spool selectively manually movable to a pair of peratin p it n cludin an xt 9- sition at which fluid motor back pressure holds the valve spool in open position until the fluid motor stroke is completed and a second position short of said extreme position at which actuation of the cylinder to a selected position may be accomplished under complete manual control.

The specific nature of this invention, as well as other objects and advantages thereof, will become apparent to those skilled in the art from the following detailed description, taken in conjunction with the attached sheets of drawings on which, by way oi preferred example only, is illustrated one embodiment of this invention.

On the drawings:

Figure 1 is a longitudinal sectional view of a control valve of the present invention forming a po o a s hem t ca r r s n e hyd lic control circuit for a fluid pressure motor;

Figure 2 is a sectional view taken along the plane 2-2 of Figure 1;

Figure 3 is a sectional view of Figure 1 illustrating the valve in a first adjusted position; and

Figure .4 is a View similar to Figures 1 and 3 illustrating the valve in a second adjusted position.

As shown on the drawings:

In Figure 1, reference numeral I0 refers generally to a control valve of the present inven tion including a valve housing I] having a longitudinal, substantially cylindrical spool bore [2 therein and a lower second bore I3 substantially axially parallel to the bore 12. A valve spool m is disposed within th ousin H an th spool 14 is effective to control the flow of fluid through the housing bores.

The cylinder housing is provided with a .pair of primary inlet passages l5 communicating with the bore [2 and with a branched pressure dis charge conduit Hi from a pump P. Preferably, the pump P is driven by a tractor or similar prime mover having a tractor operated hydrau-r lically actuated control system, indicated schematically at TR, such as the hydraulic system for a well-known type of tractor adapted for the control of tractor mounted implements.

The tractor system TR. receives fluid under pressure through ,a conduit i9 which co-mmunh cates with an outlet passage 29 opening to the bore H. The housing lower bore 13 communicates, as through passages 21, with conduits 22 leading to the fluid pressure ports '23 of .a fiuid motor, such as a double acting hydraulic cylinder 24 having an interior reciprocable piston 24a. The bore i2 also communicates with a pair of return conduits 25 communicating with the intake side of the pump P and with return passages 223 formed in the housing I l.

More specifically, the valve HE is provided with axially spaced primary inlet ports in full communication with the bore [2 for the reception of fluid under pressure from the inlet passages i5 and with a primary outlet port 3! located axially intermediate the inlet ports 30 i bore and a snap ring 38 is utilized to retain the 7 retainer in position. A washer 39 is seated against the shoulder 38 in spaced relation to a second washer at which is normally urged against a retainer 4| by a helical coil spring 2 for a purpose to be hereinafter more mully described. The retainers 3i and M are provided with annular seal rings 33 to seal the bore against the escape of fiuid under pressure.

The bore I2 slidably journals a valve spool for longitudinal axial movement therein. The spool 25 has a single centrally located primary land 66 which is centrally located within the primary outlet port 3! when the spool is centered in the bore [2. On either side of the primary land 46, the spool has axially elongated pressure grooves M which terminate at flow directing lands 43. The grooves 41 are normally efiective to establish communication between the primary inlet ports 30 and the primary outlet port- 3! so that, when the valve is in its central position, fluid flow from the pump P and the conduit i8 is merely circulated through the ports 30 and the bore into the outlet port 3! for passage through the tractor hydraulic, system TR through the line it and the port as.

The flow directing lands 68 are of a diameter substantially the same as the diameter of the bore l2, so that these lands eiiectively close off communication between the inlet ports 30 and the pressure discharge ports 32 when the valve is in its position illustrated in Figure 1. Axially outwardly of each of the flow directing lands 48, the valve spool is provided with reduced diameter return grooves 49 which normally bridge the return ports 26 and which terminate at terminal back pressure lands 5% formed on the spool and disposed within the bore axially outwardly of the return ports.

The flow directing lands 18, at their axially inner faces, are provided with pressure relieving grooves 5! for a purpose to be hereinafter more fully described. The valve spool 45 is provided with reduced diameter end portions 52 and 53 which respectively project axially beyond the bore and into the retainers 3i and 4!. The right hand spool end portion 52 is pivotally connected, as by pin 54 and link 55, to the reduced end 56 of a centrally pivoted handle 51. It will be appreciated that actuation of the handie 5? will effect reciprocation of the spool with-- in the bore [2, the direction of reciprocation depending upon the direction of handle actuation. The other end 53 of the spool 45 projects through the annular washers 39 and All and through the bore of the centralizing spring fiifi to carry a snap ring 58 against which the end washer it is seated when the washer is not seated against the retainer ll.

The lower bore it includes a central bore section 60 within which is mounted a reciprocable shuttle or cylinder 3i having reduced ends 62 projecting through the pressure discharge passages 33 which establish communication between the bores l2 and 13'. The shuttle terminates in opposed ends 65 which project through annular valve seats 54 bottomed with enlarged valve chambers 65 concentric with the lower bore 83. The valve seats 64 are centrally apertured, as at 55, for cooperationwith spherical lock valves 6'! urged against the valve seats by compression springs 58 acting on valve retainers 69 havingrelieved forward concave iacesgiil contacting the valves 67. The springs t8 are confined against the valve retainers by valve blocks H inserted into the counterbores B5 and. retained therein by annular lock rings i2.

It will be noted that the lock valves 5'! are interposed between the pressure discharge passages 33 and the cylinder discharge passages 23, so that no ilow or" fluid from the bore l2 to the cylinder 2-? can occur until the lock valves have been removed from their seats 64. Also, the

lock valves 6'5 serve to loci; fluid within the cylinder 2 5, so that a chosen position of the piston 24a will be retained until such time as the lock valves are opened and fluid is permitted to escape from the cylinder. In this connection, it will be noted that the passages 2i communicate with the interior of the valve blocks ll through grooves 13 and passages l l' located rearwardly of the lock valves El. Thus, return pressure from the cylinder 24 will not open the valves fi'l from their seats 64.

The valve spool i5 is axially counterbored, as at 15, through substantially the entire bore length, and a plug 18 is inserted in the counterbore between the two return grooves til. Each of the grooves 4 5? is radially bored, as at H, for communication with the counterbore 75, but communication between the two return grooves is prevented by the plug l6.

Theback pressure land 54} at each end of the spool is provided with an axially extending back pressure bleed groove or passage l8 (best seen in Figure 2) for a purpose to be hereinafter more fully described, and each back pressure land is provided with a plurality of radial back pressure escape orifices l9 communicating with the central counterbore '35, also as best seen in Figure 2'. In addition, the reduced end 5?. of the spool is provided with radial relief passage 86 which is immediately adjacent the land 51?. It will be noted from Figure 1 that the land 573 is spaced from the adjacent retainer 37 when the spool is in its central position of Figure l to provide a back pressure chamber 8|.

Operation As hereinbefore explained, the valve of the present invention is effective to cause either complete movement of the cylinder to the end of its stroke even though the valve handle is manually released, or to complete manual adjustment of the cylinder to a desired position in accordance with the movement of the handle. These two types of operation of the handle will be separately explained as follows:

Manually controlled operation Upon actuation of the handle 5? in a clockwise direction as viewed in Figure 1, the primary land 46 is moved beyond the primary outlet port 33 to close oil the left hand primary inlet port 30 from the primary outlet port 3|. At the same time, the right hand flow directing land 48 is moved to close off the right hand primary inlet port 36.

The feathering groove 5| in the flow directing land 48 on the right hand side of the spool serves to gradually cut off the flow of pressured fluid from the right hand port 33 to the tractor port 20, while at the same time the left hand fluid directing land 48 is being interposed between the left hand pressure port 32 and the return port 26. The feathering groove 5! on the left hand fluid directing land d8 serves to gradual- 1y establish communication between the left and primary inlet port Fill and the left hand pressure discharge port 32. Thus, by the provision of the feathering grooves 5!, close control can be maintained over the speed of piston move ment, and there is no possibility of completely shutting olT fluid pressure flow through the valve housing at any time during valve-spool movement.

When the valve spool assumes its position illustrated in Figure 3, it will be seen that the flow of fluid pressure from the pump P to the tractor system TR has been completely interrupted and the entire output of the pump is passing through the left hand branch of the conduit i6 through the left primary inlet port 39 and the left hand pressure discharge port 32. The left hand movement of the spool has compressed the centralizing spring d2 between the two washers 3 and 49 and, if the handle Si is released, the spring will return the spool to its central position at which the washer 39 abuts the terminal shoulder 36 of the bore.

Fluid pressure flow from the pressure discharge port 32 is directed downwardly through the fluid pressure passage 33 into the lower bore i3, inasmuch as the land 58 is interposed between the discharge port 32 and the return port 3d. The pressure the bore is confined to the left hand side of the freely movable shuttle 6i, and such pressure causes the shuttle to move to the right eiiecting engagement of the projection it with the right hand spherical lock valve 6"! urging the valve against the adjacent valve retainer 69 and away from the seat 64 to accommodate return fluid from the cylinder 24 as will be hereinafter described.

The same fluid pressure which is effective to move the shuttle 5! is also exerted upon the left hand spherical lock valve ti. This pressure is sufiicient to overcome the compression spring 63 and to move the left hand lock valve away from its seat 84.

Fluid flow will now be accommodated through the valve 67 and the associated passage t6, then through the cylinder line 22 to the cylinder bore 23, causing movement of the cylinder piston 24o toward the right hand side of the cylinder spool.

Movement of the cylinder piston will generate back pressure on the right hand side of the cylinder piston, inasmuch as a double acting piston of this type is always full of fluid on both sides of the piston. Fluid under this back pressure will escape through the right hand fluid conduit 22 upwardly beyond the lock valves 61 and past the shuttle projection 62 into the right hand pressure discharge passage 33. The shuttle projection 62 is slotted, as at 62a, to insure the passage of return fluid even though the shuttle projection may abut the lock valve seat 64.

Fluid from the right hand passage 33 will flow into the right hand pressure discharge port 32 and then through the return groove 49 of the spool into the right hand return port 34, the fluid passing through the right hand return conduit to the pump P. Thus, it will be appreciated that the flow of pressured fluid through the valve is under the control of the handle when the handle is actuated in accordance with the foregoing description.

Minute adjustments in actuation of the cylinder piston 24a may be made by merely cracking the valve to an open position at which communication between the primary inlet port and the associated pressure discharge port takes place only through the corresponding venting groove 5| and any degree of valve opening from a mere feathering opening to a full flow opening may be obtained. Of course, it will be appreciated that the piston 2411 may be moved to the left by merely actuating the handle 5'! in a counterclockwise direction. In either instance, the operation is the same as hereinbefore described.

V aloe holdover operation The valve it may also be utilized for holdover operation of the cylinder, that is movement of the handle to an exaggerated position will effect movement of the valve spool to such a degree that the valve is held open until the cylinder piston reaches the end of its stroke, at which time the valve will be returned to a centered position.

The movement of the valve to an extreme position is illustrated in Figure 4 of the drawings, wherein it will be noted that the valve handle has been moved so that the valve spool has been urged to the left well beyond the degree of movement illustrated in Figure 3. More particularly, the valve has been moved to the left so that the right hand back pressure land 50 completely shuts off the corresponding right hand return groove 34. Also, it will be noted that the primary land 46 is still interposed between the primary outlet port 3! and the left hand primary inlet port 30. The venting grooves 5| are well beyond the associated ports 30 and 32 so that the valve is now in its fully opened position and pump flow is being fully delivered to the left hand side of the cylinder piston 24a.

The spherical lock valves 6'! are both fully open and the flow of fluid to the left hand side of the cylinder piston is substantially the same as that described in connection with Figure 3. Return flow of fluid from the right hand side of the piston again occurs through the piston conduit 22 through the right hand lock valve 61, through the right hand .pressure discharge passage 33 and into the right hand pressure discharge port 32. Pressured fluid within the port 32 cannot escape to the tractor system TR, since the right hand flow directing land 48 is imposed between the port .32 and the port 30, nor can fluid escape through the right hand return groove 34 to the pum P, since the back pressure land 50 at the right hand end of the spool covers this port 26.

Fluid flow is thus directed through the radial by-pass port '51 in the spool into the axial back pressure passage 15. Back pressure fluid can escape from the passage 15 through the radial port 80 on the right hand side of the back pressure'land 50 into the chamber 8| defined between the back pressure land 50 and the right hand retainer 31. Fluid within this chamber 8| will exert pressure within the chamber and against the right hand face 82 of the back pressure land to urge the valve spool toward the left against action Of the compression spring 42.

t wil1 be appreciated that back pressure will be generated within the chamber 8! at any time that the back pressure land 50 fully covers the return port 34, and this pressure will urge the spool further toward the left against the action of the spring until the spring is fully compressed or until pressure is vented from the chamber 8|.

In Figure 4, the valve spool is illustrated in its furthest leftward position and it will be noted that the radial back pressure escape orifices i9 communicate with the return port 3 1, so that pressure Within the back pressure passage will be vented to the port 34 through the orifices I9. However, there will still bea substantial pressure Within the chamber 8! which is sufl'icient to hold the valve in its left hand position, since restricted communication only is possible between the escape orifices and the escape port and the back pressure generated will be sufiicient to hold the valve spool against the spring 42.

Thus, it will be seen that the valve will be held in its position toward the left against the action of the spring so long as substantial back pressure exists within the chamber 8!, i. e., so long as a substantial back pressure is generated within the cylinder. When the cylinder reaches the end of its stroke, there is no fluid on the right hand side thereof, the fluid has been exhausted back to the pump P and there is no back pressure within the passage '35 or the chamber 8! At this time, the spring 62 urges the valve from its position of Figure 4 toward its position of Figure 1. Any fluid. remaining within the chamher 8! is forced from the chamber through the bleed passage 13 into the return groove 69 and thence through the return line to the pump.

Thus, the valve spool is recentered within the housing and the valve is ready for actuation, as

desired to effect another cycle of operation of the cylinder.

Operation of the valve upon movement of the handle in a counterclockwise position is substantially the same as that hereinbefore described. Upon movement of the valve spool to the right, the spring 32 is compressed between the washer backed by the snap ring 58 and the washer 39 backed against the housing shoulder 35. When the land 5% at the left end of the spool covers the left hand port 34, holdover action occurs as hereinbefore described.

From the foregoing description of the structure and function of the present invention, it will be appreciated that the present invention provides a manually controlled valve for eiiecting actuation of a cylinder piston through either its full stroke or through a portion of the stroke. The valve may be moved to an extreme position so that the valve is automatically held open until such time as the cylinder piston stroke has been completed or the cylinder stroke may be limited under complete manual control upon movement of this invention and it is, therefore, not the purpose to limit the patent granted hereon otherwise 8 than necessitated by the scope of the appended claims.

I claim:

1. A control valve adapted to be interposed between a source of fluid pressure and a fluid motor having a cylinder and a piston reciprocable there in, comprising a valve casing having an interior bore connecting a pressure fluid inlet to a first cylinder port means on one side of said piston, said valve bore also connecting a second cylinder port means on the other side of said piston and an exhaust outlet connected to said source, a movable valve spool in said casing having an enlarged terminal portion and an enlarged land interposed between said pressure fluid inlet and said exhaust outlet when said spool is in its normal central position, means resiliently urging said spool to its central position, additional valve means normally interposed between said second cylinder port means and said exhaust outlet, said spool being manually movable from its central position to connect said pressure fluid inlet and said first cylinder port means, thereby moving said piston in said cylinder, working motor pressure being effective to actuate said additional valve means, said spool enlarged terminal portion being interposed between said second cylinder port means and said exhaust outlet, said spool having a bypass conduit accommodating the flow of exhaust fluid past said terminal portion to a portion of said bore closed by said terminal portion, said terminal portion having a restricted orifice accommodating partial flow of exhaust fluid to said exhaust outlet, thereby generating pressure upon said terminal portion acting in opposition to said resilient means to retain said spoolin its moved position so long as working pressure is present in said motor.

2. A control valve adapted to be interposed between a source of fluid pressure and a fluid motor having a cylinder and a piston reciprocable therein, comprising a valve casing having an interior bore connecting a pressure fluid inlet to a first cylinder port means on one side of said piston, said valve bore also connecting a second cylinder port means on the other side of said piston to an exhaust outlet connected to said source, an axially movable valve spool in said bore having an enlarged terminal portion and an enlarged land interposed between said pressure fluid inlet and said exhaust outlet when said spool is in its normal central position, spring means contacting one end of said spool in opposed relation to said enlarged portion resiliently urging said spool to its central position, additional valve means normally interposed between said second cylinder port means and said exhaust outlet, said spool being manually movable from its central position against the spring means to connect said pressure fluid inlet and said first cylinder port means, thereby moving said piston in said cylinder, working motor pressure being effective to actuate said additional valve means, said enlarged terminal portion being interposed between said second cylinder port means and said exhaust outlet, said spool having an axial by-pass conduit accommodating the flow of exhaust fluid past said terminal portion to a portion of said bore closed by said terminal portion, said terminal portion having a restricted orifice registerable with said exhaust outlet accommodating only limited flow of exhaust fluid to said exhaust outlet, thereby generating pressure upon said terminal portion acting in opposition to said spring means to retain said spool in its moved posi- 9 tion so long as working pressure is present in said motor.

3. A control valve adapted to be interposed between a source of fluid pressure and a fluid motor having a cylinder and a piston reciprocable therein, comprising a valve casing having an interior bore connecting a pressure fluid inlet to a first cylinder port means on one side of said piston, said valve bore also connecting a second cylinder port means on the other side of said piston and an exhaust outlet connected to said source, a movable valve spool in said casing having an enlarged terminal portion and an enlarged land interposed between said pressure fluid inlet and exhaust outlet when said spool'is in its normal central position, mean resiliently urging said spool to its central position, additional valve means normally interposed between said second cylinder port means and said exhaust outlet, said spool being manually movable from its central position to connect said pressure fluid inlet and said first cylinder port means, thereby moving said piston in said cylinder, working motor pressure being effective to actuate said additional valve means, said enlarged terminal portion being interposed between said second cylinder port means and said exhaust outlet, said spool having a by-pass conduit accommodating the flow of exhaust fluid past said terminal portion to a portion of said bore closed by said terminal portion, said terminal portion having a restricted orifice accommodating only restricted flow of exhaust fluid to said exhaust outlet, thereby generating pressure upon said terminal portion acting in opposition to said resilient means to retain said spool in its moved position so long as working pressure is present in said motor, and said enlarged terminal portion being provided with an aperture communicating with the spool by-pass conduit and registerable with said exhaust outlet only after said spool has been moved by said pressure acting on said terminal portion, thereby accommodating full flow of exhaust fluid while holding said spool in opposition to said resilient means.

4. A control valve adapted to be interposed between a source of fluid pressure and a double acting fluid motor having a reciprocable piston comprising a valve housing having a cylindrical interior bore connecting a pressure fluid inlet to a pair of cylinder ports on opposing sides, respectively, of said piston, said valve bore also connecting said pair of cylinder ports to exhaust outlets connected to said source, a valve spool axially movable in said bore having radially enlarged lands interposed between said pressure fluid inlet and said cylinder ports, respectively, and having radially enlarged terminal heads fitting snugly in said bore, said spool having separate axial by-pass passages at the opposite ends thereof bridging said terminal heads, respectively, resilient means biasing said spool to a central position in said bore, means for moving said spool axially in said bore to place one of said cylinder ports in communication with said pressure fluid inlet to subject one side of said motor piston to pressured fluid and to generate exhaust pressure on the other side of said piston due to piston movement, movement of said spool taking place against the biasing force of said resilient means and spool movement preventing exhaust fluid flow to the exhaust outlet normally in communication with said other side of said piston by positioning the corresponding one of said terminal heads across said port, so that flow of exhaust fluid occurs through the by-pass passage bridging said one terminal head and the exhaust fluid pressure is exerted on said one head to hold said spool in its moved position against the biasing force of said resilient means, and said head having a vent passage registerable with the associated exhaust outlet only after the subjection of the head to exhaust fluid pressure to accommodate the escape of exhaust fluid through said exhaust outlet.

5. A control valve adapted to be interposed between a source of fluid pressure and a fluid motor having a reeiprocable piston, comprising a valve casing having a cylindrical interior bore connecting a pressure fluid inlet to a first cylinder port on one side of said piston, said valve bore also connecting a second cylinder port on the other side of said piston and an exhaust outlet connected to said source, a generally cylindrical movable valve spool in said casing having an enlarged terminal portion defining an end face at one end of the spool and an enlarged land interposed between said pressure fluid inlet and said cylinder port when said valve is closed, spring means acting on the other end of the spool resiliently urging said spool to the position at which said valve is closed, additional valve means normally interposed between said second cylinder port and said exhaust outlet, said spool being manually movable from its closed position to connect said pressure fluid inlet and cylinder port, thereby moving said piston in said cylinder, piston movement generating exhaust pressure in said fluid motor effective to actuate said additional valve means to normally establish communication between said second cylinder port and said exhaust outlet, said enlarged terminal portion being interposed between said second cylinder port and said exhaust outlet, a by-pass passage accommodating the flow of exhaust fluid past said terminal portion to a portion of said bore closed by said terminal portion and communicating with the end face of the spool, said spool having a restricted orifice accommodating only restricted flow of exhaust fluid past said terminal portion to said exhaust outlet, thereby generating pressure upon said end face acting in opposition to said resilient means to retain said spool in its moved position so long as exhaust pressure is generated by piston movement, and said enlarged portion is provided with an aperture communicating with the spool by-pass conduit and registerable with said exhaust outlet only after said spool has been moved by said pressure acting on said end face, thereby accommodating full flow of exhaust fluid while holding said spool in opposition to said resilient means.

6. A control valve adapted to be interposed between a source of fluid pressure and a double acting fluid motor having a reciprocable piston, comprising a valve housing having a cylindrical interior bore connecting a pressure fluid inlet to a pair of cylinder ports on opposing sides, respectively, of said piston, said valve bore also connecting said pair of cylinder ports with exhaust outlets connected to said source, a valve spool axially movable in said bore having radially enlarged lands interposed between said pressure fluid inlet and said cylinder ports, respectively, and having radially enlarged terminal heads fitting snugly in said bore to define exhaust pressure chambers at either end of said bore, said spool having separate axial by-pass passages at the opposite ends thereof bridging said terminal heads, respectively, resilient means biasing said 1 l spool to a central position in said bore, means for moving said spool axially in said bore against the bias of said resilient means to place one of said cylinder ports in communication with said pressure fluid inlet to subject one side of said motor piston to pressured fluid and to generate exhaust pressure on the other side of said piston due to piston movement, spool movement closing the exhaust outlet in communication with the other side of said piston through the other of said cylinder ports by positioning the corresponding one of said terminal heads across said outlet, so that flow of exhaust fluid into the adjacent exhaust pressure chamber occurs through the by-pass passage bridging said one terminal head, the exhaust pressure in said one chamber acting on said head to hold said spool in its moved position against the biasing force of said resilient means, and said head having a vent passage registerable with the associated exhaust outlet only after the subjection of the head to exhaust pressure to accommodate the escape of pressure from said exhaust pressure chamber to said exhaust outlet.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,448,557 Stephens Sept. '7, 1948 2,483,312 Clay Sept. 27, 1949 2,541,958 Deardorfi Feb. 13, 1951 2,588,520 Halgren Mar. 11, 1952 

